Results of Overfeeding a Chihuhua

It's hard to find the right balance when feeding chihuahuas. In one month he has gone from under 3 pounds to 6 lbs. 5 ounces. Of course this is normal for a growing puppy. My main concern is that he eats leaves and wood chips when out in the yard and then throws them up hours later. Even when I have him on a leash he is very quick to snatch these up without me seeing him. He seems to prefer this to his dog food. I'm not sure how to break him of this habit. When I do catch him with one I take it from his mouth and loudly say NO, then toss it back onto the ground. He seems to not want to heed this command though.

Here is a picture of an obese chihuahua. It's not Oscar, btw!

Image by Wm Jas via Flickr